When performing electrolysis in the field of metal recovery from aqueous solutions, it is essential for an adequate circulation to take place at the surfaces of the electrodes, as explained below.
When an electrode is dipped into an electrolyte, a so-called phase boundary layer usually forms. It is known in theory that first an inner boundary layer, the so-called inner Helmholtz surface, is formed, which is nearest to the electrode. It is formed mainly from the molecules of the solvent. Solvated ions can only penetrate through to this first boundary layer. They themselves thereby form a second layer, the so-called outer Helmholtz surface. The interaction between the charged metal of the electrode and the solvated ions takes place only via long-ranged electrostatic forces. A layer is therefore formed away from the electrode and into the solution, which is called the diffusion layer. The thickness of this layer depends on the ion concentration of the solution. This electrode/liquid boundary layer can be aptly described as a capacitor. As in a capacitor, here too the capacitance and the charge are linked to a voltage. If a current is now passed through an electrochemical cell, then this voltage at least must be exceeded before any current flow can come about. If there is then a current flow through a cell, the metal is deposited onto the cathode. The transfer of metal ions to the cathode is caused by three different effects:
a) migration under the influence of the electrical field; PA1 b) diffusion through the concentration gradient; and PA1 c) convection by circulation of the electrolyte liquid at the electrode, surface. PA1 flow monitors, which for example trigger an alarm when the flow in the supply or discharge lines to the electrolysis cell is interrupted; PA1 flow meters, for defining a minimum throughput rate below which an alarm is triggered; PA1 thermal flow monitors; or PA1 inductive flow meters, for obtaining a precise and dependable measurement of the throughput.
In metal recovery by means of electrolysis, the metal ions are now taken away from the vicinity of the cathode and deposited as metal atoms when the appropriate charge is received. This results in a depletion of these metal ion species near the cathode. If the convection largely fails, then the processes a) and b) are substantially responsible for ion transport. Both processes generally transport the ions in a substantially less effective way than convection. A boundary layer of increasing thickness is formed in which the concentration of the ions to be deposited decreases and leads to an increase in the potential with a constant current. If the potential is kept constant, the electrolysis current drops, and hence by a considerable extent the metal recovery rate too. If, however, the current is injected from the outside as a constant current, then other ions assume the charge transport, and unwelcome oxidation and reduction processes occur at the electrodes. Furthermore, the efficiency of the metal recovery process drops even further.
The recovery of silver from photographic fixing bath solution is taken as an example here. When a current is switched on, the silver ions are deposited on the cathode as solid silver metal while absorbing an electron. With the three processes described above, new silver ions are brought to the cathode. If convection fails, i.e. no liquid flow at the cathode surface, this leads to a depletion of silver ions at the cathode surface. If the outer potential is kept constant, the current drops and the recovery rate is considerably reduced. If the current is kept constant, then the potential increases and unwelcome chemical reactions result, such as the formation of silver sulfide, the oxidation of sulfite at the anode or sulfur precipitation. In the final analysis, the fixing bath becomes unusable, which is not tolerable for silver recovery with a circulation system. The circulation of the electrolyte at the electrode surface therefore results in a reduction of the diffusion layer. The better the circulation, the better the metal deposition on the cathode. The fundamental electrochemical effects are described in, for example: "Electrochemical Methods", A. J. Bard, L. R. Faulkner, John Wiley & Sons, New York, 1980.
According to the prior art, there is either no monitoring of the electrolyte circulation (high risk) or the monitoring is performed indirectly by mechanical or electromechanical equipment such as:
Generating the convection in the electrolysis cell using a circulation pump and monitoring it in various ways is also known.
For example, an interface for a silver recovery unit for detecting the operating state of at least one pump is known from DE 195 09 757 A1. A sensor for detection of the operating state of the pump is installed outside the pump such that during the installation any interference with the electronic control unit of the pump or with the pump itself is avoidable. The output signal of the sensor is passed to a downstream electronic circuit comprising a series connection of an amplifier means, a first filter means, a rectifier means, a second filter means and a comparator.
All these methods have in common that they do not inquire about the presence or absence of a flow on the spot, i.e. directly at the electrode. Furthermore, the aforementioned mechanical or electromechanical monitoring equipment is expensive and therefore often not usable for simple silver recovery equipment.